In table-per-class strategy, for each sub entity class a separate table is generated.
Unlike joined strategy, no separate table is generated for parent entity class in table-per-class strategy.
The following syntax represents the table-per-class strategy: -
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
Table-per-class Example
In this example, we will categorize employees into active employees and retired employees.
Thus, the subclass ActiveEmployees and RetiredEmployees inherits the e_id and e_name fields of parent class Employee.
Now, follow the below steps to create JPA project,
Step 1. Create a root entity class Employee.java under the com.javahubpoint.jpa.inheritence package and specify all the required variables and annotations.
Employee.java
import java.io.Serializable;
import javax.persistence.*;
@Entity
@Table(name="employee_details")
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
public class Employee implements Serializable {
@Id
private int e_id;
private String e_name;
public Employee(int e_id, String e_name) {
super();
this.e_id = e_id;
this.e_name = e_name;
}
public Employee() {
super();
}
public int getE_id() {
return e_id;
}
public void setE_id(int e_id) {
this.e_id = e_id;
}
public String getE_name() {
return e_name;
}
public void setE_name(String e_name) {
this.e_name = e_name;
}
}
Step 2. Create an entity class ActiveEmployee.java (subclass of Employee.java) under com.javahubpoint.jpa.inheritence package.
ActiveEmployee.java
import javax.persistence.*;
@Entity
public class ActiveEmployee extends Employee {
private int e_salary;
private int e_experience;
public ActiveEmployee(int e_id, String e_name, int e_salary, int e_experience) {
super(e_id, e_name);
this.e_salary = e_salary;
this.e_experience = e_experience;
}
public ActiveEmployee() {
super();
}
public int getE_salary() {
return e_salary;
}
public void setE_salary(int e_salary) {
this.e_salary = e_salary;
}
public int getE_experience() {
return e_experience;
}
public void setE_experience(int e_experience) {
this.e_experience = e_experience;
}
}
Step 3. Create another entity class RetiredEmployee.java (subclass of Employee.java) under com.javahubpoint.jpa.inheritence package.
RetiredEmployee.java
import javax.persistence.*;
@Entity
public class RetiredEmployee extends Employee {
private int e_pension;
public RetiredEmployee(int e_id, String e_name, int e_pension) {
super(e_id, e_name);
this.e_pension = e_pension;
}
public RetiredEmployee() {
super();
}
public int getE_pension() {
return e_pension;
}
public void setE_pension(int e_pension) {
this.e_pension = e_pension;
}
}
Step 4. Now, map the entity class and other databases configuration in Persistence.xml file.
Persistence.xml
<persistence-unit name="Employee_details">
<class>com.javahubpoint.jpa.inheritence.ActiveEmployee</class>
<class>com.javahubpoint.jpa.inheritence.RetiredEmployee</class>
<class>com.javahubpoint.jpa.inheritence.Employee</class>
<properties>
<property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/employee"/>
<property name="javax.persistence.jdbc.user" value="root"/>
<property name="javax.persistence.jdbc.password" value=""/>
<property name="eclipselink.logging.level" value="SEVERE"/>
<property name="eclipselink.ddl-generation" value="create-or-extend-tables"/>
</properties>
</persistence-unit>
</persistence>
Step 5. Create a persistence class EmployeePersistence.java under com.javahubpoint.jpa.persistence package. This class is used to initialize an object and persist it.
EmployeePersistence.java
import javax.persistence.*;
import com.javahubpoint.jpa.inheritence.*;
public class EmployeePersistence {
public static void main(String[] args) {
EntityManagerFactory emf=Persistence.createEntityManagerFactory("Employee_details");
EntityManager em=emf.createEntityManager();
em.getTransaction().begin();
ActiveEmployee ae1=new ActiveEmployee(101,"Karun",10000,5);
ActiveEmployee ae2=new ActiveEmployee(102,"Rishab",12000,7);
RetiredEmployee re1=new RetiredEmployee(103,"Ramesh",5000);
RetiredEmployee re2=new RetiredEmployee(104,"Raj",4000);
em.persist(ae1);
em.persist(ae2);
em.persist(re1);
em.persist(re2);
em.getTransaction().commit();
em.close();
emf.close();
}}
Output:
After the execution of the program, two tables are generated in MySQL workbench.
Now, fetch data from each table separately to generate the output.
Select * from active_employee
Select * from retired_employee
